1. Field of the Invention
The present invention relates to current sensors which detect current in a wide measurement range with high precision and with low power consumption.
2. Description of the Related Art
For example, current sensors measure the magnitude of current for driving motors in electric cars. The current sensors include a magnetic proportional current sensor (open-loop current sensor) and a magnetic balance current sensor (closed-loop current sensor). In a magnetic proportional current sensor, a magnetic field which is proportional to measurement current occurs in a core gap due to magnetic lines of force which are produced in a core made from a magnetic substance, and a magnetic detection device converts this magnetic field into a voltage signal to produce an output voltage that is proportional to the measurement current. In contrast, in a magnetic balance current sensor, when measurement current flows, a magnetic detection device produces an output voltage due to a magnetic field depending on the current. The magnetic detection device outputs a voltage signal which is converted into current so that the current is fed back to a feedback coil. The feedback coil produces a magnetic field (canceling magnetic field), and the canceling magnetic field and the magnetic field produced by the measurement current cancel each other out so that the resulting magnitude of the magnetic fields is constantly equal to zero. The current sensor converts the feedback current which flows through the feedback coil at that time into a voltage to output the resulting voltage.
A magnetoresistive element, such as a Hall element or a giant magneto resistance (GMR) element, is used as a magnetic detection device in the above-described current sensor. A magnetic proportional current sensor using a Hall element as a magnetic detection device, when used in a wider measurement range, has a low resolution for a small measurement current. In contrast, a magnetic balance current sensor using a Hall element as a magnetic detection device fails to cancel the magnetic field produced by a large amount of current. Thus, a method for compensating for the drawbacks of the two types of current sensors is disclosed in Japanese Unexamined Patent Application Publication No. 2007-78416 which describes a method in which a magnetic proportional current sensor using a Hall element and a magnetic balance current sensor using a Hall element are disposed so as to be used by switching therebetween in accordance with the magnitude of measurement current.
However, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-78416, it is necessary to prepare two types of current sensors separately, resulting in failure to achieve space saving and in a complicated manufacturing process. Furthermore, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-78416, in the case where the measurement current is large, magnetic saturation occurs when a GMR element is used as a magnetic detection device because the magnetic balance is disturbed in the magnetic balance current sensor which is not being used. Thus, in the case where the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-78416 is used, some magnetic detection devices do not enable measurement to be performed in a wide measurement range with high precision.